1. Field of the Invention
This invention relates to an electro-chromic display in which reversible changes in color are obtained by application of an electrical field to an electro-chromic material and which is, for example, utilized for a digital display watch or a portable electronic calculator.
2. Description of the Prior Art
An electro-chromic display generally comprises a display electrode formed by a combination of segments, a counter electrode positioned opposite to the display electrode and defining a passage for electric current therebetween, and an electro-chromic material and an electrolyte provided in the passage for electric current, and accomplishes coloration and discoloration reversibly upon application of an electrical field.
In the event the counter electrode is formed from tungsten oxide by evaporation or sputtering (and the display electrode is also made of tungsten oxide without being limited thereto), it has a chemical composition which is very close to that of WO.sub.3 and permits no further oxidation. In order that the reversible reaction takes place, tungsten oxide counter electrode should be kept in a partially reduced state. For this purpose, it would be necessary to apply such high voltage as to be composed the electrolyte at the beginnings of the operation, or else be necessary to employ a technique taught in U.S. Pat. No. 3,840,287. Then, tungsten oxide will change as follows, EQU WO.sub.3 +xM.sup.+ +xe.sup.- .fwdarw.M.sub.x WO.sub.3,
where x is a value from 0 to 1, M stands H, Li, Na or other metal and M.sub.x WO.sub.3 is the same chemical formula as that of tungsten bronze. The parameter x denotes the degree of reduction, and it increases in the reduction while it decreases in the oxidation. When an electro-chromic display device is operated to color the display electrode, x decreases as the result of the oxidation at the counter electrode, and with the decoloration, it increases as the result of the oxidation. If a value of x is small, the counter electrode cannot afford sufficient charge to conduct the coloration operation. Moreover the equilibrium potential of M.sub.x WO.sub.3 depends largely on the value of x. For further details, see, for example, "Physical Review" B, 16, 1750 (1977), or "Journal of Electrochemical Society," 125, 1603 (1978).
Thus, it follows that if the device is operated at a "constant voltage driving" by application of a constant voltage for a pre-determined length of time across the display electrode and the counter electrode opposite thereto, the potential applied to the display electrode depends on the value x of the counter electrode, and thereby influences the density of the coloration developed on the display electrode. This may, for example, bring about the following disadvantages ("uneven coloration"):
(i) If a coloration pulse is applied to one segment after another, a later activated segment will have a lower degree of color density; and
(ii) if a plurality of segments are driven simultaneously, their color density will become lower with an increase in the total area of the display electrodes which are driven simultaneously.
It is known to construct a cell by employing a greater value of x, as taught in U.S. Pat. No. 3,840,287. Even if this method is adopted, however, the oxygen dissolved in the electrolyte or the oxygen in the air entering the cell through its sealed portion causes the value of x to decrease with the lapse of time. It is, therefore, difficult to keep the value of x at at least 0.1. This tendency becomes more evident with an increase in the temperature of the environment in which the cell is situated, at thin film of tungsten oxide.
An increase in the charge capacity of the counter electrode at the fixed value of x may be expected by increasing the volume of tungsten oxide on the counter electrode. But it is not practical solution for the above-mentioned problem by the following reasons. If the counter electrode is too large as compared with the display electrode, however, it destroys a balance in the design of the display device. Moreover, it is difficult to obtain a sufficiently large film thickness by an ordinary method of evaporation and thick film would cause a voltage drop across its thickness. According to the conventional practice, a paste or fluid obtained by kneading powders of manganese oxide and carbon with a binder is coated on a conductor to form the counter electrode. The binder is selected from among oranic polymer compounds such as an epoxy, silicone or phenolic resin. The counter electrode prepared in accordance with the prior art, however, has a high degree of density polarization, and a high level of specific resistance. Accordingly, the device has a slow rate of response for coloration and discoloration, and moreover, the potential of the manganese oxide lacks stability. This is due to the fact that as the manganese oxide and carbon powders are covered with the binder, the manganese oxide has a lowered degree of activity, and the carbon powder has an increased degree of interfacial resistance.
In order to solve these problems, it has been proposed to use tar, pitch or theriac as the binder, and fire a mixture of manganese oxide and carbon powders and the binder at a temperature of one thousand and several hundred degrees centigrade to form a plate defining the counter electrode. The use of such a high temperature is, however, likely to result in the decomposition and phase change of manganese oxide, and disables its stability in potential to be advantageously manifested.